1. Field of the Invention
The present invention relates to a light emitting device that includes LEDs emitting short-wavelength visible light and a fluorescent material absorbing the visible light and emitting fluorescent light of a longer wavelength than the absorbed light, and emits white light or intermediate-color light. The present invention also relates to a method for manufacturing the light emitting device.
2. Related Art
In recent years, attention is drawn to so-called white LEDs that are formed by combining blue light emitting diodes (LED) with a yellow fluorescent material such as YAG doped with Ce. Each of such white LEDs emits white light with a single chip. Conventionally, LEDs emit light in a single color, such as red, green, or blue. To emit white light or intermediate-color light, LEDs that emit single-color light need to be used and driven. At present, however, such trouble is eliminated by combining light emitting diodes and a fluorescent material. Thus, white light can be obtained with a simple structure.
There has also been a suggested light emitting device that has a large number of LED chips mounted on a mounting substrate, and convex lenses formed at the portions where the blue LED chips are mounted. The convex lenses are formed with a light transmissive resin containing a diffused fluorescent material that absorbs blue light emitted from the blue LED chips and emit yellow light (see JP-A 2001-148514 (KOKAI), for example). In the light emitting device disclosed in JP-A 2001-148514 (KOKAI), the compounding ratio of the fluorescent material to the resin is determined in advance. Therefore, the compounding ratio of the fluorescent material cannot be adjusted in accordance with the light emission wavelengths of the LED chips, and variations in emission color cannot be reduced.
To reduce the variations in emission color, a light emitting device that has a color conversion material placed on each lens has been suggested. The color conversion material is molded into a dome-like shape, and is made of a light transmissive resin containing a fluorescent material diffused at compounding ratios varied in accordance with the emission wavelength of the LED chips (see Japanese Patent Publication No. 3,941,826, for example). In the light emitting device disclosed in Japanese Patent Publication No. 3,941,826, however, an air layer is provided between the lens and the color conversion material. Therefore, light loss due to total reflection at the interface between the lens and the color conversion material containing a fluorescent material cannot be prevented, and it is difficult to output high-intensity light with high luminance efficiency. Furthermore, with the air layer, reductions in the light transmission rate of the resin and the luminance efficiency of the fluorescent material cannot be avoided.
To counter this problem, a light emitting device that outputs high-intensity light with high luminance efficiency has been suggested as a structure having LED chips and fluorescent layers mounted thereon without an air layer (see JP-A 2007-273562 (KOKAI), for example). The light emitting device disclosed in JP-A 2007-273562 (KOKAI) has a dome-like multi-layer structure that includes: a substrate that has a flat face on which a semiconductor light emitting element is to be mounted; the semiconductor light emitting element that is mounted on the flat face of the substrate, and emits light within the range of ultraviolet light to visible light; a first light transmissive layer that is formed on the substrate and covers the semiconductor light emitting element; a fluorescent material layer that is formed on the first light transmissive layer, has an end portion reaching the flat face of the substrate, and contains a fluorescent material and a base material; and a second light transmissive layer that is formed on the fluorescent material layer, and has an end portion reaching the flat face of the substrate. However, the light emitting device disclosed in JP-A 2007-273562 (KOKAI) is formed on a flat-type mounting substrate. Therefore, it is difficult to provide particular distributions to the emission patterns.
As described above, a conventional light emitting device that can output white light has an air layer between each lens and a color conversion material. Therefore, light loss due to total reflection at the interface between the lens and the color conversion material containing a fluorescent material cannot be prevented, and it is difficult to output high-intensity light with high luminance efficiency. Furthermore, with the air layer, the transmission rate of the resin and the luminance efficiency of the fluorescent material might deteriorate. Also, since the mounting substrate for the LEDs is of a flat type, it is difficult to provide particular distributions to emission patterns.